Method for feeding back reference signal information in multi-antenna wireless communication system and apparatus therefor

ABSTRACT

The present invention relates to a method and apparatus for feeding back, by a terminal, reference signal information in wireless communication using a two-dimensional active antenna system including a plurality of antennas. Particularly, the method for feeding back reference signal information comprises the steps of: receiving, from a base station, a reference signal configuration including identifiers of a plurality of reference signals having a first reference signal set and a second reference signal set; receiving the plurality of reference signals to which the precoding is applied; measuring reference signal received power (RSRP) for each of the plurality of reference signals; and transmitting, to the base station, information on at least a part of the first reference signal set and information on at least a part of the second reference signal set, on the basis of the measured RSRP. In addition, the precoding for the terminal is determined on the basis of the information on at least a part of the first reference signal set, and the interference information for the terminal can be determined on the basis of the information on at least a part of the second reference signal set.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Phase of PCT International ApplicationNo. PCT/KR2016/005031, filed on May 12, 2016, which claims priorityunder 35 U.S.C. 119(e) to U.S. Provisional Application No. 62/161,914,filed on May 15, 2015, all of which are hereby expressly incorporated byreference into the present application.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method for feeding back reference signalinformation in a multi-antenna wireless communication system andapparatus therefor.

BACKGROUND ART

Multiple input multiple output (MIMO) increases the efficiency of datatransmission and reception using multiple transmit antennas and multiplereceive antennas instead of a single transmission antenna and a singlereception antenna. A receiver receives data through multiple paths whenmultiple antennas are used, whereas the receiver receives data through asingle antenna path when a single antenna is used. Accordingly, MIMO canincrease a data transmission rate and throughput and improve coverage.

A single cell MIMO scheme can be classified into a single user-MIMO(SU-MIMO) scheme for receiving a downlink signal by a single UE in onecell and a multi user-MIMO (MU-MIMO) scheme for receiving a downlinksignal by two or more UEs.

Channel estimation refers to a procedure for compensating for signaldistortion due to fading to restore a reception signal. Here, the fadingrefers to sudden fluctuation in signal intensity due to multipath-timedelay in a wireless communication system environment. For channelestimation, a reference signal (RS) known to both a transmitter and areceiver is required. In addition, the RS can be referred to as a RS ora pilot signal according to applied standard.

A downlink RS is a pilot signal for coherent demodulation for a physicaldownlink shared channel (PDSCH), a physical control format indicatorchannel (PCFICH), a physical hybrid indicator channel (PHICH), aphysical downlink control channel (PDCCH), etc. A downlink RS includes acommon RS (CRS) shared by all user equipments (UEs) in a cell and adedicated RS (DRS) for a specific UE. For a system (e.g., a systemhaving extended antenna configuration LTE-A standard for supporting 8transmission antennas) compared with a conventional communication system(e.g., a system according to LTE release-8 or 9) for supporting 4transmission antennas. DRS based data demodulation has been consideredfor effectively managing RSs and supporting a developed transmissionscheme. That is, for supporting data transmission through extendedantennas. DRS for two or more layers can be defined. DRS is pre-coded bythe same pre-coder as a pre-coder for data and thus a receiver caneasily estimate channel information for data demodulation withoutseparate precoding information.

A downlink receiver can acquire pre-coded channel information forextended antenna configuration through DRS but requires a separate RSother than DRS in order to non-pre-coded channel information.Accordingly, a receiver of a system according to LTE-A standard candefine a RS for acquisition of channel state information (CSI), that is,CSI-RS.

DISCLOSURE OF THE INVENTION Technical Task

The technical task of the present invention is to provide a method forfeeding back reference signal information in a wireless communicationsystem and apparatus therefor

It will be appreciated by persons skilled in the art that the objectsthat could be achieved with the present invention are not limited towhat has been particularly described hereinabove and the above and otherobjects that the present invention could achieve will be more clearlyunderstood from the following detailed description.

Technical Solutions

To achieve these objects and other advantages, in an aspect of thepresent invention, provided herein is a method for feeding backreference signal information by a user equipment (UE) in wirelesscommunication using a two-dimensional active antenna system (2D-AAS)including multiple antennas, including: receiving, from a base station(BS), a reference signal configuration including identifiers (IDs) of aplurality of reference signals included in a first reference signal setand a second reference signal set; receiving the plurality of referencesignals where precoding is applied; measuring reference signal receivedpower (RSRP) for each of the plurality of reference signals; andtransmitting, to the BS, information on at least part of the firstreference signal set and information on at least part of the secondreference signal set based on the measured RSRP. In this case, precodingfor the UE may be determined based on the information on the at leastpart of the first reference signal set, and interference information onthe UE may be determined based on the information on the at least partof the second reference signal set.

Additionally, the first reference signal set may be composed ofreference signals of the BS, and the second reference signal set may becomposed of reference signals of a neighboring BS adjacent to the BS.

Additionally, different precoding may be applied to each of thereference signals in the first reference signal set such that each ofthe reference signals in the first reference signal set has a differentincidence angle in a vertical direction.

Additionally, the information on the at least part of the firstreference signal set may include an ID(s) of at least one referencesignal having RSRP equal to or greater than a first threshold value inthe first reference signal set, and the information on the at least partof the second reference signal set may include an ID(s) of at least onereference signal having RSRP equal to or greater than a second thresholdvalue in the second reference signal set.

Additionally, the information on the at least part of the firstreference signal set may further include RSRP for the at least onereference signal having the RSRP equal to or greater than the firstthreshold value in the first reference signal set, and the informationon the at least part of the second reference signal set may furtherinclude RSRP for the at least one reference signal having the RSRP equalto or greater than the second threshold value in the second referencesignal set.

Additionally, the information on the at least part of the firstreference signal set may include an ID(s) of at least one referencesignal having RSRP equal to or greater than a first threshold value inthe first reference signal set, and the information on the at least partof the second reference signal set may include an ID(s) of at least onereference signal having RSRP equal to or greater than a second thresholdvalue in the second reference signal set.

Additionally, a time period for measuring RSRP for each of a pluralityof reference signals in the first reference signal set may be set to belonger than that for measuring RSRP for each of a plurality of referencesignals in the second reference signal set.

Additionally, the method may further include: determining a referencesignal that is not successfully received based on the reference signalconfiguration; and transmitting an ID of the determined referencesignal, which is not successfully received, to the BS.

In another aspect of the present invention, provided herein is a userequipment (UE) in wireless communication using a two-dimensional activeantenna system (2D-AAS) including multiple antennas, including: atransceiver configured to transmit and receive signals; and a processorconfigured to control the transceiver, wherein the processor is furtherconfigured to: receive, from a base station (BS), a reference signalconfiguration including identifiers (IDs) of a plurality of referencesignals included in a first reference signal set and a second referencesignal set; receive the plurality of reference signals where a pluralityof pieces of precoding is applied respectively; measure reference signalreceived power (RSRP) for each of the plurality of reference signals;and transmit, to the BS, information on at least part of the firstreference signal set and information on at least part of the secondreference signal set based on the measured RSRP. In this case, precodingfor the UE may be determined based on the information on the at leastpart of the first reference signal set, and interference information onthe UE may be determined based on the information on the at least partof the second reference signal set.

Advantageous Effects

According to embodiments of the present invention, it is possible toprovide an efficient method for feeding back a reference signal in amulti-antenna system and apparatus therefor.

It will be appreciated by persons skilled in the art that that theeffects achieved by the present invention are not limited to what hasbeen particularly described hereinabove and other advantages of thepresent invention will be more clearly understood from the followingdetailed description.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention:

FIG. 1 is a diagram showing a network structure of an Evolved UniversalMobile Telecommunications System (E-UMTS) as an example of a wirelesscommunication system;

FIG. 2 is a block diagram illustrating configurations of a base station205 and a user equipment 210 in a wireless communication system 200according to the present invention;

FIG. 3 is a diagram for a configuration of a general MIMO communicationsystem;

FIG. 4 is a diagram for an example of a general CDD structure in a MIMOsystem;

FIG. 5 is a diagram illustrating legacy CRS and DRS patterns;

FIG. 6 is a diagram illustrating an example of a DM RS pattern:

FIG. 7 is a diagram illustrating examples of a CSI-RS pattern;

FIG. 8 is a diagram for explaining an example of a scheme ofperiodically transmitting a CSI-RS;

FIG. 9 is a diagram for explaining an example of a scheme ofaperiodically transmitting a CSI-RS;

FIG. 10 is a diagram for explaining an example of using two CSI-RSconfigurations;

FIG. 11 illustrates an active antenna system (AAS).

FIG. 12 illustrates an example of measurement using a precoded CSI-RS.

FIG. 13 is a flowchart for explaining a method for feeding back areference signal according to an embodiment of the present invention.

BEST MODE FOR INVENTION

Hereinafter, the preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. It is to beunderstood that the detailed description, which will be disclosed alongwith the accompanying drawings, is intended to describe the exemplaryembodiments of the present invention, and is not intended to describe aunique embodiment with which the present invention can be carried out.The following detailed description includes detailed matters to providefull understanding of the present invention. However, it will beapparent to those skilled in the art that the present invention can becarried out without the detailed matters. For example, the followingdetailed description is given under the assumption that 3GPP LTE mobilecommunication systems are used. However, the description may be appliedto any other mobile communication system except for specific featuresinherent to the 3GPP LTE systems.

In some cases, to prevent the concept of the present invention frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, wherever possible, thesame reference numbers will be used throughout the drawings and thespecification to refer to the same or like parts.

Moreover, in the following description, it is assumed that a terminalrefers to a mobile or fixed type user equipment such as a user equipment(UE), and an advanced mobile station (AMS). Also, it is assumed that abase station refers to a random node of a network terminal, such as NodeB, eNode B, and an access point (AP), which performs communication withthe user equipment.

In a mobile communication system, a user equipment may receiveinformation from a base station through a downlink and transmitinformation to the base station through an uplink. The information thatthe user equipment transmits or receives includes data and various typesof control information. There are various physical channels according tothe types and usages of information that the user equipment transmits orreceives.

As an example of a mobile communication system to which the presentinvention is applicable, a 3rd Generation Partnership Project Long TermEvolution (hereinafter, referred to as LTE) communication system isdescribed in brief.

FIG. 1 is a view schematically illustrating a network structure of anE-UMTS as an exemplary radio communication system.

An Evolved Universal Mobile Telecommunications System (E-UMTS) is anadvanced version of a conventional Universal Mobile TelecommunicationsSystem (UMTS) and basic standardization thereof is currently underway inthe 3GPP. E-UMTS may be generally referred to as a Long Term Evolution(LTE) system. For details of the technical specifications of the UMTSand E-UMTS, reference can be made to Release 7 and Release 8 of “3rdGeneration Partnership Project; Technical Specification Group RadioAccess Network”.

Referring to FIG. 1, the E-UMTS includes a User Equipment (UE), eNode Bs(eNBs), and an Access Gateway (AG) which is located at an end of thenetwork (E-UTRAN) and connected to an external network. The eNBs maysimultaneously transmit multiple data streams for a broadcast service, amulticast service, and/or a unicast service.

One or more cells may exist per eNB. The cell is set to operate in oneof bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides adownlink (DL) or uplink (UL) transmission service to a plurality of UEsin the bandwidth. Different cells may be set to provide differentbandwidths. The eNB controls data transmission or reception to and froma plurality of UEs. The eNB transmits DL scheduling information of DLdata to a corresponding UE so as to inform the UE of a time/frequencydomain in which the DL data is supposed to be transmitted, coding, adata size, and hybrid automatic repeat and request (HARQ)-relatedinformation.

In addition, the eNB transmits UL scheduling information of UL data to acorresponding UE so as to inform the UE of a time/frequency domain whichmay be used by the UE, coding, a data size, and HARQ-relatedinformation. An interface for transmitting user traffic or controltraffic may be used between eNBs. A core network (CN) may include the AGand a network node or the like for user registration of UEs. The AGmanages the mobility of a UE on a tracking area (TA) basis. One TAincludes a plurality of cells.

Although wireless communication technology has been developed to LTEbased on wideband code division multiple access (WCDMA), the demands andexpectations of users and service providers are on the rise. Inaddition, considering other radio access technologies under development,new technological evolution is required to secure high competitivenessin the future. Decrease in cost per bit, increase in serviceavailability, flexible use of frequency bands, a simplified structure,an open interface, appropriate power consumption of UEs, and the likeare required.

Recently, 3GPP has standardized technology subsequent to LTE. In thisspecification, the technology will be referred to as “LTE-Advanced” or“LTE-A”. A main difference between the LTE system and the LTE-A systemis a system bandwidth. The LTE-A system aims to support a wideband of upto 100 MHz. To achieve this, the LTE-A system employs carrieraggregation or bandwidth aggregation that accomplishes a wideband usinga plurality of frequency blocks. Carrier aggregation uses a plurality offrequency blocks as a large logical frequency band in order to achieve awider frequency band. The bandwidth of each frequency block can bedefined on the basis of a system block bandwidth used in the LTE system.Each frequency block is transmitted using a component carrier.

FIG. 2 is a block diagram illustrating configurations of a base station205 and a user equipment 210 in a wireless communication system 200.

Although one base station 205 and one user equipment 210 are shown forsimplification of a wireless communication system 200, the wirelesscommunication system 200 may include one or more base stations and/orone or more user equipments.

Referring to FIG. 2, the base station 105 may include a transmitting(Tx) data processor 215, a symbol modulator 220, a transmitter 225, atransmitting and receiving antenna 230, a processor 280, a memory 285, areceiver 290, a symbol demodulator 295, and a receiving (Rx) dataprocessor 297. The user equipment 210 may include a Tx data processor265, a symbol modulator 270, a transmitter 275, a transmitting andreceiving antenna 235, a processor 255, a memory 260, a receiver 240, asymbol demodulator 255, and an Rx data processor 250. Although theantennas 230 and 235 are respectively shown in the base station 205 andthe user equipment 210, each of the base station 205 and the userequipment 210 includes a plurality of antennas. Accordingly, the basestation 205 and the user equipment 210 according to the presentinvention support a multiple input multiple output (MIMO) system. Also,the base station 205 according to the present invention may support botha single user-MIMO (SU-MIMO) system and a multi user-MIMO (MU-MIMO)system.

On a downlink, the Tx data processor 215 receives traffic data, formatsand codes the received traffic data, interleaves and modulates (orsymbol maps) the coded traffic data, and provides the modulated symbols(“data symbols”). The symbol modulator 220 receives and processes thedata symbols and pilot symbols and provides streams of the symbols.

The symbol modulator 220 multiplexes the data and pilot symbols andtransmits the multiplexed data and pilot symbols to the transmitter 225.At this time, the respective transmitted symbols may be a signal valueof null, the data symbols and the pilot symbols. In each symbol period,the pilot symbols may be transmitted continuously. The pilot symbols maybe frequency division multiplexing (FDM) symbols, orthogonal frequencydivision multiplexing (OFDM) symbols, time division multiplexing (TDM)symbols, or code division multiplexing (CDM) symbols.

The transmitter 225 receives the streams of the symbols and converts thereceived streams into one or more analog symbols. Also, the transmitter225 generates downlink signals suitable for transmission through a radiochannel by additionally controlling (for example, amplifying, filteringand frequency upconverting) the analog signals. Subsequently, thedownlink signals are transmitted to the user equipment through theantenna 230.

In the user equipment 210, the antenna 235 receives the downlink signalsfrom the base station 205 and provides the received signals to thereceiver 240. The receiver 240 controls (for example, filters, amplifiesand frequency downcoverts) the received signals and digitalizes thecontrolled signals to acquire samples. The symbol demodulator 245demodulates the received pilot symbols and provides the demodulatedpilot symbols to the processor 255 to perform channel estimation.

Also, the symbol demodulator 245 receives a frequency responseestimation value for the downlink from the processor 255, acquires datasymbol estimation values (estimation values of the transmitted datasymbols) by performing data demodulation for the received data symbols,and provides the data symbol estimation values to the Rx data processor250. The Rx data processor 250 demodulates (i.e., symbol de-mapping),deinterleaves, and decodes the data symbol estimation values to recoverthe transmitted traffic data.

Processing based on the symbol demodulator 245 and the Rx data processor250 is complementary to processing based on the symbol demodulator 220and the Tx data processor 215 at the base station 205.

On an uplink, the Tx data processor 265 of the user equipment 210processes traffic data and provides data symbols. The symbol modulator270 receives the data symbols, multiplexes the received data symbolswith the pilot symbols, performs modulation for the multiplexed symbols,and provides the streams of the symbols to the transmitter 275. Thetransmitter 275 receives and processes the streams of the symbols andgenerates uplink signals. The uplink signals are transmitted to the basestation 205 through the antenna 235.

The uplink signals are received in the base station 205 from the userequipment 210 through the antenna 230, and the receiver 290 processesthe received uplink signals to acquire samples. Subsequently, the symboldemodulator 295 processes the samples and provides data symbolestimation values and the pilot symbols received for the uplink. The Rxdata processor 297 recovers the traffic data transmitted from the userequipment 210 by processing the data symbol estimation values.

The processors 255 and 280 of the user equipment 210 and the basestation 205 respectively command (for example, control, adjust, manage,etc.) the operation at the user equipment 210 and the base station 205.The processors 255 and 280 may respectively be connected with thememories 260 and 285 that store program codes and data. The memories 260and 285 respectively connected to the processor 280 store operatingsystem, application, and general files therein.

Each of the processors 255 and 280 may be referred to as a controller, amicrocontroller, a microprocessor, and a microcomputer. Meanwhile, theprocessors 255 and 280 may be implemented by hardware, firmware,software, or their combination. If the embodiment of the presentinvention is implemented by hardware, application specific integratedcircuits (ASICs), digital signal processors (DSPs), digital signalprocessing devices (DSPDs), programmable logic devices (PLDs), and fieldprogrammable gate arrays (FPGAs) configured to perform the embodiment ofthe present invention may be provided in the processors 255 and 280.Meanwhile, if the embodiment according to the present invention isimplemented by firmware or software, firmware or software may beconfigured to include a module, a procedure, or a function, whichperforms functions or operations of the present invention. Firmware orsoftware configured to perform the present invention may be provided inthe processors 255 and 280, or may be stored in the memories 260 and 285and driven by the processors 255 and 280.

Layers of a radio interface protocol between the user equipment 110 orthe base station 105 and a wireless communication system (network) maybe classified into a first layer L1, a second layer L2 and a third layerL3 on the basis of three lower layers of OSI (open systeminterconnection) standard model widely known in communication systems. Aphysical layer belongs to the first layer L1 and provides an informationtransfer service using a physical channel. A radio resource control(RRC) layer belongs to the third layer and provides control radioresources between the user equipment and the network. The user equipmentand the base station may exchange RRC messages with each another throughthe RRC layer.

The term, base station used in the present invention may refer to a“cell or sector” when used as a regional concept. A serving base station(or serving cell) may be regarded as a base station which provides mainservices to UEs and may transmit and receive control information on acoordinated multiple transmission point. In this sense, the serving basestation (or serving cell) may be referred to as an anchor base station(or anchor cell). Likewise, a neighboring base station may be referredto as a neighbor cell used as a local concept.

Multiple Antenna System

In the multiple antenna technology, reception of one whole message doesnot depend on a single antenna path. Instead, in the multiple antennatechnology, data fragments received through multiple antennas arecollected and combined to complete data. If the multiple antennatechnology is used, a data transfer rate within a cell region of aspecific size may be improved, or system coverage may be improved whileensuring a specific data transfer rate. In addition, this technology canbe broadly used by mobile communication devices and relays. Due to themultiple antenna technology, restriction on mobile communication trafficbased on a legacy technology using a single antenna can be solved.

FIG. 3(a) shows the configuration of a wireless communication systemincluding multiple antennas. As shown in FIG. 3(a), the number oftransmit (Tx) antennas and the number of Rx antennas respectively toN_(T) and N_(R), a theoretical channel transmission capacity of the MIMOcommunication system increases in proportion to the number of antennas,differently from the above-mentioned case in which only a transmitter orreceiver uses several antennas, so that transmission rate and frequencyefficiency can be greatly increased. In this case, the transfer rateacquired by the increasing channel transmission capacity cantheoretically increase by a predetermined amount that corresponds tomultiplication of a maximum transfer rate (Ro) acquired when one antennais used and a rate of increase (Ri). The rate of increase (Ri) can berepresented by the following equation 1.R _(i)=min(N _(T) , N _(R))   [Equation 1]

For example, provided that a MIMO system uses four Tx antennas and fourRx antennas, the MIMO system can theoretically acquire a high transferrate which is four times higher than that of a single antenna system.After the above-mentioned theoretical capacity increase of the MIMOsystem was demonstrated in the mid-1990s, many developers began toconduct intensive research into a variety of technologies which cansubstantially increase data transfer rate using the theoretical capacityincrease. Some of the above technologies have been reflected in avariety of wireless communication standards, for example,third-generation mobile communication or next-generation wireless LAN,etc.

A variety of MIMO-associated technologies have been intensivelyresearched by many companies or developers, for example, research intoinformation theory associated with MIMO communication capacity undervarious channel environments or multiple access environments, researchinto a radio frequency (RF) channel measurement and modeling of the MIMOsystem, and research into a space-time signal processing technology.

Mathematical modeling of a communication method for use in theabove-mentioned MIMO system will hereinafter be described in detail. Ascan be seen from FIG. 7, it is assumed that there are N_(T) Tx antennasand N_(R) Rx antennas. In the case of a transmission signal, a maximumnumber of transmission information pieces is N_(T) under the conditionthat N_(T) Tx antennas are used, so that the transmission informationcan be represented by a specific vector shown in the following equation2.S=[S₁, S₂, . . . , S_(N) _(T) ]^(T)   [Equation 2]

In the meantime, individual transmission information pieces s₁, s₂, . .. , s_(NT) may have different transmission powers. In this case, if theindividual transmission powers are denoted by P₁, P₂, . . . , P_(NT),transmission information having an adjusted transmission power can berepresented by a specific vector shown in the following equation 3.Ŝ=└Ŝ₁, Ŝ₂, . . . , Ŝ_(N) _(T) ┘^(T)=[PS₁, PS₂, . . . , PS_(N) _(T) ]^(T)  [Equation 3]

In Equation 3, Ŝ is a transmission vector, and can be represented by thefollowing equation 4 using a diagonal matrix P of a transmission power.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In the meantime, the information vector Ŝ having an adjustedtransmission power is applied to a weight matrix W, so that N_(T)transmission signals x₁, x₂, . . . , x_(NT) to be actually transmittedare configured. In this case, the weight matrix W is adapted to properlydistribute transmission information to individual antennas according totransmission channel situations. The above-mentioned transmissionsignals x₁, x₂, . . . , x_(NT) can be represented by the followingequation 5 using the vector X. Here, W_(ij) denotes a weightcorresponding to i-th Tx antenna and j-th information. W represents aweight matrix or precoding matrix.

$\begin{matrix}{x = {\quad{\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix} = {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{12} & w_{12} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 2} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\;\hat{s}} = {WPs}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Given N_(R) Rx antennas, signals received at the respective Rx antennas,y₁, y₂, . . . , y_(N) _(R) may be represented as the following vector.y=[y₁, y₂, . . . , y_(N) _(R) ]^(T)   [Equation 6]

When channels are modeled in the MIMO communication system, they may bedistinguished according to the indexes of Tx and Rx antennas and thechannel between a j^(th) Tx antenna and an i^(th) Rx antenna may berepresented as h_(ij). It is to be noted herein that the index of the Rxantenna precedes that of the Tx antenna in h_(ij).

The channels may be represented as vectors and matrices by groupingthem. Examples of vector expressions are given as below. FIG. 3(b)illustrates channels from N_(T) Tx antennas to an i^(th) Rx antenna.

As illustrated in FIG. 3(b), the channels from the N_(T) Tx antennas toan i^(th) Rx antenna may be expressed as follows.h_(i) ^(T)=[h_(i1), h_(i2), . . . , h_(iN) _(T) ]  [Equation 7]

Also, all channels from the N_(T) Tx antennas to the N_(R) Rx antennasmay be expressed as the following matrix.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{12} & h_{12} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 2} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

Actual channels experience the above channel matrix H and then are addedwith Additive White Gaussian Noise (AWGN). The AWGN n₁, n₂, . . . ,n_(N) _(R) added to the N_(R) Rx antennas is given as the followingvector.n=[n₁, n₂, . . . , n_(N) _(R) ]^(T)  [Equation 9]

From the above modeled equations, the received signal can be expressedas follows.

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {{{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{12} & h_{12} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 2} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}} = {{Hx} + n}}}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

In the meantime, the numbers of rows and columns in the channel matrix Hrepresenting channel states are determined according to the numbers ofTx and Rx antennas. The number of rows is identical to that of Rxantennas, N_(R) and the number of columns is identical to that of Txantennas, N_(T). Thus, the channel matrix H is of size N_(R)×N_(T). Ingeneral, the rank of a matrix is defined as the smaller between thenumbers of independent rows and columns. Accordingly, the rank of thematrix is not larger than the number of rows or columns. The rank of thematrix H, rank(H) is limited as follows.rank(H)≤min(N _(T) , N _(R))  [Equation 11]

As a multi-antenna transmission and reception scheme used for operatinga multi-antenna system, it may be able to use FSTD (frequency switchedtransmit diversity), SFBC (Space Frequency Block Code), STBC (Space TimeBlock Code), CDD (Cyclic Delay Diversity), TSTD (time switched transmitdiversity) and the like. In a rank 2 or higher, SM (SpatialMultiplexing), GCDD (Generalized Cyclic Delay Diversity), S-VAP(Selective Virtual Antenna Permutation) and the like can be used.

The FSTD corresponds to a scheme of obtaining a diversity gain byassigning a subcarrier of a different frequency to a signal transmittedby each of multiple antennas. The SFBC corresponds to a scheme capableof securing both a diversity gain in a corresponding dimension and amulti-user scheduling gain by efficiently applying selectivity in aspatial domain and a frequency domain. The STBC corresponds to a schemeof applying selectivity in a spatial domain and a time domain. The CDDcorresponds to a scheme of obtaining a diversity gain using path delaybetween transmission antennas. The TSTD corresponds to a scheme ofdistinguishing signals transmitted by multiple antennas from each otheron the basis of time. The spatial multiplexing (SM) corresponds to ascheme of increasing a transfer rate by transmitting a different dataaccording to an antenna. The GCDD corresponds to a scheme of applyingselectivity in a time domain and a frequency domain. The S-VAPcorresponds to a scheme of using a single precoding matrix. The S-VAPcan be classified into an MCW (multi codeword) S-VAP for mixing multiplecodewords between antennas in spatial diversity or spatial multiplexingand an SCW (single codeword) S-VAP for using a single codeword.

Among the aforementioned MIMO transmission schemes, the STBC schemecorresponds to a scheme of obtaining time diversity in a manner that anidentical data symbol is repeated in a time domain to supportorthogonality. Similarly, the SFBC scheme corresponds to a scheme ofobtaining frequency diversity in a manner that an identical data symbolis repeated in a frequency domain to support orthogonality. Examples ofa time block code used for the STBC and a frequency block code used forthe SFBC can be represented as equation 12 and equation 13,respectively. The equation 12 indicates a block code in case of 2transmission antennas and the equation 13 indicates a block code in caseof 4 transmission antennas.

$\begin{matrix}{\frac{1}{\sqrt{2}}\begin{pmatrix}S_{1} & S_{2} \\{- S_{2}^{*}} & S_{1}^{*}\end{pmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 12} \right\rbrack \\{\frac{1}{\sqrt{2}}\begin{pmatrix}S_{1} & S_{2} & 0 & 0 \\0 & 0 & S_{3} & S_{4} \\{- S_{2}^{*}} & S_{1}^{*} & 0 & 0 \\0 & 0 & {- S_{4}^{*}} & S_{3}^{*}\end{pmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 13} \right\rbrack\end{matrix}$

In the equations 12 and 13, Si (i=1, 2, 3, 4) corresponds to a modulateddata symbol. And, in the equations 12 and 13, a row of a matrixcorresponds to an antenna port and a column of the matrix corresponds totime (STBC) or frequency (SFBC).

Meanwhile, among the aforementioned MIMO transmission schemes, the CDDscheme corresponds to a scheme of increasing frequency diversity byincreasing delay propagation on purpose. FIG. 4 shows an example of ageneral CDD structure in a multi-antenna system. FIG. 4(a) shows ascheme of applying cyclic delay in time domain. As shown in FIG. 4(b),the CDD scheme applying the cyclic delay of FIG. 4(a) can also beimplemented by applying phase-shift diversity.

Reference Signals (RSs)

In a wireless communication system, a packet is transmitted on a radiochannel. In view of the nature of the radio channel, the packet may bedistorted during the transmission. To receive the signal successfully, areceiver should compensate for the distortion of the reception signalusing channel information. Generally, to enable the receiver to acquirethe channel information, a transmitter transmits a signal known to boththe transmitter and the receiver and the receiver acquires knowledge ofchannel information based on the distortion of the signal received onthe radio channel. This signal is called a pilot signal or an RS.

In the case of data transmission and reception through multipleantennas, knowledge of channel states between transmission (Tx) antennasand reception (Rx) antennas is required for successful signal reception.Accordingly, an RS should be transmitted through each Tx antenna.

RSs in a mobile communication system may be divided into two typesaccording to their purposes: RS for channel information acquisition andRS for data demodulation. Since its purpose lies in that a UE acquiresdownlink channel information, the former should be transmitted in abroad band and received and measured even by a UE that does not receivedownlink data in a specific subframe. This RS is also used in asituation like handover. The latter is an RS that an eNB transmits alongwith downlink data in specific resources. A UE can estimate a channel byreceiving the RS and accordingly can demodulate data. The RS should betransmitted in a data transmission area.

A legacy 3GPP LTE (e.g., 3GPP LTE release-8) system defines two types ofdownlink RSs for unicast services: a common RS (CRS) and a dedicated RS(DRS). The CRS is used for acquisition of information about a channelstate, measurement of handover, etc. and may be referred to as acell-specific RS. The DRS is used for data demodulation and may bereferred to as a UE-specific RS. In a legacy 3GPP LTE system, the DRS isused for data demodulation only and the CRS can be used for bothpurposes of channel information acquisition and data demodulation.

CRSs, which are cell-specific, are transmitted across a wideband inevery subframe. According to the number of Tx antennas at an eNB, theeNB may transmit CRSs for up to four antenna ports. For instance, an eNBwith two Tx antennas transmits CRSs for antenna port 0 and antenna port1. If the eNB has four Tx antennas, it transmits CRSs for respectivefour Tx antenna ports, antenna port 0 to antenna port 3.

FIG. 5 illustrates a CRS and DRS pattern for an RB (including 14 OFDMsymbols in time by 12 subcarriers in frequency in case of a normal CP)in a system where an eNB has four Tx antennas. In FIG. 5, REs labeledwith ‘R0’, ‘R1’, ‘R2’ and ‘R3’ represent the positions of CRSs forantenna port 0 to antenna port 4, respectively. REs labeled with ‘D’represent the positions of DRSs defined in the LTE system.

The LTE-A system, an evolution of the LTE system, can support up toeight Tx antennas. Therefore, it should also support RSs for up to eightTx antennas. Because downlink RSs are defined only for up to four Txantennas in the LTE system, RSs should be additionally defined for fiveto eight Tx antenna ports, when an eNB has five to eight downlink Txantennas in the LTE-A system. Both RSs for channel measurement and RSsfor data demodulation should be considered for up to eight Tx antennaports.

One of significant considerations for design of the LTE-A system isbackward compatibility. Backward compatibility is a feature thatguarantees a legacy LTE terminal to operate normally even in the LTE-Asystem. If RSs for up to eight Tx antenna ports are added to atime-frequency area in which CRSs defined by the LTE standard aretransmitted across a total frequency band in every subframe, RS overheadbecomes huge. Therefore, new RSs should be designed for up to eightantenna ports in such a manner that RS overhead is reduced.

Largely, new two types of RSs are introduced to the LTE-A system. Onetype is CSI-RS serving the purpose of channel measurement for selectionof a transmission rank, a modulation and coding scheme (MCS), aprecoding matrix index (PMI), etc. The other type is demodulation RS (DMRS) for demodulation of data transmitted through up to eight Txantennas.

Compared to the CRS used for both purposes of measurement such aschannel measurement and measurement for handover and data demodulationin the legacy LTE system, the CSI-RS is designed mainly for channelestimation, although it may also be used for measurement for handover.Since CSI-RSs are transmitted only for the purpose of acquisition ofchannel information, they may not be transmitted in every subframe,unlike CRSs in the legacy LTE system. Accordingly, CSI-RSs may beconfigured so as to be transmitted intermittently (e.g. periodically)along the time axis, for reduction of CSI-RS overhead.

When data is transmitted in a downlink subframe, DM RSs are alsotransmitted dedicatedly to a UE for which the data transmission isscheduled. Thus, DM RSs dedicated to a particular UE may be designedsuch that they are transmitted only in a resource area scheduled for theparticular UE, that is, only in a time-frequency area carrying data forthe particular UE.

FIG. 6 illustrates an exemplary DM RS pattern defined for the LTE-Asystem. In FIG. 6, the positions of REs carrying DM RSs in an RBcarrying downlink data (an RB having 14 OFDM symbols in time by 12subcarriers in frequency in case of a normal CP) are marked. DM RSs maybe transmitted for additionally defined four antenna ports, antenna port7 to antenna port 10 in the LTE-A system. DM RSs for different antennaports may be identified by their different frequency resources(subcarriers) and/or different time resources (OFDM symbols). This meansthat the DM RSs may be multiplexed in frequency division multiplexing(FDM) and/or time division multiplexing (TDM). If DM RSs for differentantenna ports are positioned in the same time-frequency resources, theymay be identified by their different orthogonal codes. That is, these DMRSs may be multiplexed in Code Division Multiplexing (CDM). In theillustrated case of FIG. 6, DM RSs for antenna port 7 and antenna port 8may be located on REs of DM RS CDM group 1 through multiplexing based onorthogonal codes. Similarly, DM RSs for antenna port 9 and antenna port10 may be located on REs of DM RS CDM group 2 through multiplexing basedon orthogonal codes.

FIG. 7 illustrates exemplary CSI-RS patterns defined for the LTE-Asystem. In FIG. 7, the positions of REs carrying CSI-RSs in an RBcarrying downlink data (an RB having 14 OFDM symbols in time by 12subcarriers in frequency in case of a normal CP) are marked. One of theCSI-RS patterns illustrated in FIGS. 7(a) to 7(e) is available for anydownlink subframe. CSI-RSs may be transmitted for eight antenna portssupported by the LTE-A system, antenna port 15 to antenna port 22.CSI-RSs for different antenna ports may be identified by their differentfrequency resources (subcarriers) and/or different time resources (OFDMsymbols). This means that the CSI-RSs may be multiplexed in FDM and/orTDM. CSI-RSs positioned in the same time-frequency resources fordifferent antenna ports may be identified by their different orthogonalcodes. That is, these DM RSs may be multiplexed in CDM. In theillustrated case of FIG. 7(a), CSI-RSs for antenna port 15 and antennaport 16 may be located on REs of CSI-RS CDM group 1 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 17 and antenna port18 may be located on REs of CSI-RS CDM group 2 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 19 and antenna port20 may be located on REs of CSI-RS CDM group 3 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 21 and antenna port22 may be located on REs of CSI-RS CDM group 4 through multiplexingbased on orthogonal codes. The same principle described with referenceto FIG. 7(a) is applicable to the CSI-RS patterns illustrated in FIGS.7(b) to 7(e).

RS patterns shown in FIGS. 5 to 7 are disclosed only for illustrativepurposes, and the scope or spirit of the present invention are notlimited only to a specific RS pattern. That is, even in the case inwhich RS patterns different from those of FIGS. 5 to 7 are defined andused, various embodiments of the present invention can also be equallyapplied thereto without difficulty.

CSI-RS Configuration

Among a plurality of CSI-RSs and a plurality of IMRs set to a UE, oneCSI process can be defined in a manner of associating a CSI-RS resourcefor measuring a signal with an interference measurement resource (IMR)for measuring interference. A UE feedbacks CSI information induced fromCSI processes different from each other to a network (e.g., basestation) with an independent period and a subframe offset.

In particular, each CSI process has an independent CSI feedbackconfiguration. The base station can inform the UE of the CS-RS resource,the IMR resource association information and the CSI feedbackconfiguration via higher layer signaling. For example, assume that threeCSI processes shown in Table 1 are set to the UE.

TABLE 1 Signal Measurement CSI Process Resource (SMR) IMR CSI process 0CSI-RS 0 IMR 0 CSI process 1 CSI-RS 1 IMR 1 CSI process 2 CSI-RS 0 IMR 2

In Table 1, a CSI-RS 0 and a CSI-RS 1 indicate a CSI-RS received from acell 1 corresponding to a serving cell of a UE and a CSI-RS receivedfrom a cell 2 corresponding to a neighbor cell participating incooperation, respectively. IMRs set to each of the CSI processes shownin Table 1 are shown in Table 2.

TABLE 2 IMR eNB 1 eNB 2 IMR 0 Muting Data transmission IMR 1 Datatransmission Muting IMR 2 Muting Muting

A cell 1 performs muting in an IMR 0 and a cell 2 performs datatransmission in the IMR 0. A UE is configured to measure interferencefrom other cells except the cell 1 in the IMR 0. Similarly, the cell 2performs muting in an IMR 1 and the cell 1 performs data transmission inthe IMR 1. The UE is configured to measure interference from other cellsexcept the cell 2 in the IMR 1. The cell 1 and the cell 2 perform mutingin an IMR 2 and the UE is configured to measure interference from othercells except the cell 1 and the cell 2 in the IMR 2.

Hence, as shown in Table 1 and Table 2, if data is received from thecell 1, CSI information of the CSI process 0 indicates optimized RI, PMIand CQI information. If data is received from the cell 2, CSIinformation of the CSI process 1 indicates optimized RI, PMI and CQIinformation. If data is received from the cell 1 and there is nointerference from the cell 2, CSI information of the CSI process 2indicates optimized RI, PMI and CQI information.

It is preferable for a plurality of CSI processes set to a UE to sharevalues subordinate to each other. For example, in case of jointtransmission performed by the cell 1 and the cell 2, if a CSI process 1considering a channel of the cell 1 as a signal part and a CSI process 2considering a channel of the cell 2 as a signal part are set to a UE, itis able to easily perform JT scheduling only when ranks of the CSIprocess 1 and the CSI process 2 and a selected subband index areidentical to each other.

A period or a pattern of transmitting a CSI-RS can be configured by abase station. In order to measure the CSI-RS, a UE should be aware ofCSI-RS configuration of each CSI-RS antenna port of a cell to which theUE belongs thereto. The CSI-RS configuration can include a DL subframeindex in which the CSI-RS is transmitted, time-frequency location of aCSI-RS resource element (RE) in a transmission subframe (e.g., theCSI-RS patterns shown in FIGS. 7(a) to 7(e)) and a CSI-RS sequence (asequence used for a CSI-RS usage, the sequence is pseudo-randomlygenerated according to a prescribed rule based on a slot number, a cellID, a CP length and the like), etc. In particular, a plurality of CSI-RSconfigurations can be used by a random (given) base station and the basestation can inform a UE(s) in a cell of a CSI-RS configuration to beused for the UE(s).

Since it is necessary to identify a CSI-RS for each antenna port,resources to which the CSI-RS for each antenna port is transmittedshould be orthogonal to each other. As mentioned earlier with referenceto FIG. 7, the CSI-RS for each antenna port can be multiplexed by theFDM, the TDM and/or the CDM scheme using an orthogonal frequencyresource, an orthogonal time resource and/or an orthogonal coderesource.

When the base station informs the UEs in a cell of information on aCSI-RS (CSI-RS configuration), it is necessary for the base station topreferentially inform the UEs of information on time-frequency to whichthe CSI-RS for each antenna port is mapped. Specifically, information ontime can include numbers of subframes in which a CSI-RS is transmitted,a period of transmitting a CSI-RS, a subframe offset of transmitting aCSI-RS, an OFDM symbol number in which a CSI-RS resource element (RE) ofa specific antenna is transmitted, etc. Information on frequency caninclude a frequency space of transmitting a CSI-RS resource element (RE)of a specific antenna, an RE offset on a frequency axis, a shift value,etc.

FIG. 8 is a diagram for explaining an example of a scheme ofperiodically transmitting a CSI-RS. A CSI-RS can be periodicallytransmitted with a period of an integer multiple of a subframe (e.g.,5-subframe period, 10-subframe period, 20-subframe period, 40-subframeperiod or 80-subframe period).

FIG. 8 shows a radio frame configured by 10 subframes (subframe number 0to 9). In FIG. 8, for example, a transmission period of a CSI-RS of abase station corresponds to 10 ms (i.e., 10 subframes) and a CSI-RStransmission offset corresponds to 3. The offset value may varydepending on a base station to make CSI-RSs of many cells to be evenlydistributed in time domain. If a CSI-RS is transmitted with a period of10 ms, an offset value may have one selected from among 0 to 9.Similarly, if a CSI-RS is transmitted with a period of 5 ms, an offsetvalue may have one selected from among 0 to 4. If a CSI-RS istransmitted with a period of 20 ms, an offset value may have oneselected from among 0 to 19. If a CSI-RS is transmitted with a period of40 ms, an offset value may have one selected from among 0 to 39. If aCSI-RS is transmitted with a period of 80 ms, an offset value may haveone selected from among 0 to 79. The offset value corresponds to a valueof a subframe in which CSI-RS transmission starts by a base stationtransmitting a CSI-RS with a prescribed period. If the base stationinforms a UE of a transmission period of a CSI-RS and an offset value,the UE is able to receive the CSI-RS of the base station at acorresponding subframe position using the transmission period and theoffset value. The UE measures a channel through the received CSI-RS andmay be then able to report such information as a CQI, a PMI and/or an RI(rank indicator) to the base station. In the present disclosure, theCQI, the PMI and/or the RI can he commonly referred to as CQI (or CSI)except a case of individually explaining the CQI, the PMI and/or the RI.And, the CSI-RS transmission period and the offset can be separatelydesignated according to a CSI-RS configuration.

FIG. 9 is a diagram for explaining an example of a scheme ofaperiodically transmitting a CSI-RS. In FIG. 9, for example, one radioframe is configured by 10 subframes (subframe number 0 to 9). As shownin FIG. 9, a subframe in which a CSI-RS is transmitted can berepresented as a specific pattern. For example, a CSI-RS transmissionpattern can be configured by a 10-subframe unit and whether to transmita CSI-RS can be indicated by a 1-bit indicator in each subframe. Anexample of FIG. 9 shows a pattern of transmitting a CSI-RS in a subframeindex 3 and 4 among 10 subframes (subframe index 0 to 9). The indicatorcan be provided to a UE via higher layer signaling.

As mentioned in the foregoing description, configuration of CSI-RStransmission can be variously configured. In order to make a UE properlyreceive a CSI-RS and perform channel measurement, it is necessary for abase station to inform the UE of CSI-RS configuration. Embodiments ofthe present invention for informing a UE of CSI-RS configuration areexplained in the following.

Method of Indicating CSI-RS Configuration

In general, a base station is able to inform a UE of CSI-RSconfiguration by one of two schemes in the following.

A first scheme is a scheme that a base station broadcasts information onCSI-RS configuration to UEs using dynamic broadcast channel (DBCH)signaling.

In a legacy LTE system, when a base station informs UEs of contents onsystem information, the information is transmitted to the UEs via a BCH(broadcasting channel). Yet, if the contents are too much and the BCH isunable to carry all of the contents, the base station transmits thesystem information using a scheme used for transmitting a generaldownlink data. And, PDCCH CRC of corresponding data is transmitted in amanner of being masked using SI-RNTI, i.e., system information RNTI,instead of a specific UE ID (e.g., C-RNTI). In this case, actual systeminformation is transmitted to a PDSCH region together with a generalunicast data. By doing so, all UEs in a cell decode PDCCH using theSI-RNTI, decode PDSCH indicated by the corresponding PDCCH and may bethen able to obtain the system information. This type of broadcastingscheme may be referred to as a DBCH (dynamic BCH) to differentiate itfrom a general broadcasting scheme, i.e., PBCH (physical BCH).

Meanwhile, system information broadcasted in a legacy LTE system can bedivided into two types. One is a master information block (MIB)transmitted on the PBCH and another one is a system information block(SIB) transmitted on a PDSCH region in a manner of being multiplexedwith a general unicast data. In the legacy LIE system, sinceinformations transmitted with an SIB type 1 to an SIB type 8 (SIB1 toSIB8) are already defined, it may be able to define a new SIB type totransmit information on a CSI-RS configuration corresponding to newsystem information not defined in the legacy SIB types. For example, itmay be able to define SIB9 or SIB10 and the base station can inform UEswithin a cell of the information on the CSI-RS configuration via theSIB9 or the SIB10 using a DBCH scheme.

A second scheme is a scheme that a base station informs each UE ofinformation on CSI-RS configuration using RRC (radio resource control)signaling. In particular, the information on the CSI-RS can be providedto each of the UEs within a cell using dedicated RRC signaling. Forexample, in the course of establishing a connection with the basestation via an initial access or handover of a UE, the base station caninform the UE of the CSI-RS configuration via RRC signaling. Or, whenthe base station transmits an RRC signaling message, which requireschannel status feedback based on CSI-RS measurement, to the UE, the basestation can inform the UE of the CSI-RS configuration via the RRCsignaling message.

Indication of CSI-RS Configuration

A random base station may use a plurality of CSI-RS configurations andthe base station can transmit a CSI-RS according to each of a pluralityof the CSI-RS configurations to a UE in a predetermined subframe. Inthis case, the base station informs the UE of a plurality of the CSI-RSconfigurations and may be able to inform the UE of a CSI-RS to be usedfor measuring a channel state for making a feedback on a CQI (channelquality information) or CSI (channel state information).

Embodiments for a base station to indicate a CSI-RS configuration to beused in a UE and a CSI-RS to be used for measuring a channel areexplained in the following.

FIG. 10 is a diagram for explaining an example of using two CSI-RSconfigurations. In FIG. 10, for example, one radio frame is configuredby 10 subframes (subframe number 0 to 9). In FIG. 10, in case of a firstCSI-RS configuration, i.e., a CSI-RS a transmission period of a CSI-RSis 10 ms and a transmission offset of a CSI-RS is 3. In FIG. 10, in caseof a second CSI-RS configuration, i.e., a CSI-RS2, a transmission periodof a CSI-RS is 10 ms and a transmission offset of a CSI-RS is 4. A basestation informs a UE of information on two CSI-RS configurations and maybe able to inform the UE of a CSI-RS configuration to be used for CQI(or CSI) feedback among the two CSI-RS configurations.

If the base station asks the UE to make a CQI feedback on a specificCSI-RS configuration, the UE can perform channel state measurement usinga CSI-RS belonging to the CSI-RS configuration only. Specifically, achannel state is determined based on CSI-RS reception quality, an amountof noise/interference and a function of a correlation coefficient. Inthis case, the CSI-RS reception quality is measured using the CSI-RSbelonging to the CSI-RS configuration only. In order to measure theamount of noise/interference and the correlation coefficient (e.g., aninterference covariance matrix indicating interference direction, etc.),measurement can be performed in a subframe in which the CSI-RS istransmitted or a subframe designated in advance. For example, in theembodiment of FIG. 10, if the base station asks the UE to make afeedback on the first CSI-RS configuration (CSI-RSI), the UE measuresreception quality using a CSI-RS transmitted in a fourth subframe (asubframe index 3) of a radio frame and the UE can be separatelydesignated to use an add number subframe to measure the amount ofnoise/interference and the correlation coefficient. Or, it is able todesignate the UE to measure the CSI-RS reception quality, the amount ofnoise/interference and the correlation coefficient in a specific singlesubframe (e.g., a subframe index 3) only.

For example, reception signal quality measured using a CSI-RS can besimply represented by SINR (signal-to-interference plus noise ratio) asS/(I+N) (in this case, S corresponds to strength of a reception signal,I corresponds to an amount of interference and N corresponds to anamount of noise). The S can be measured through a CSI-RS in a subframeincluding the CSI-RS in a subframe including a signal transmitted to aUE. Since the I and the N change according to an amount of interferencereceived from a neighbor cell, direction of a signal received from aneighbor cell, and the like, the I and the N can be measured by a CRStransmitted in a subframe in which the S is measured or a separatelydesignated subframe, etc.

In this case, the amount of noise/interference and the correlationcoefficient can be measured in a resource element (RE) in which a CRSbelonging to a corresponding subframe or a CSI-RS is transmitted. Or, inorder to easily measure noise/interference, the noise/interference canbe measured through a configured null RE. In order to measurenoise/interference in a CRS or CSI-RS RE, a UE preferentially recovers aCRS or a CSI-RS and subtracts a result of the recovery from a receptionsignal to make a noise and interference signal to be remained only. Bydoing so, the UE is able to obtain statistics of noise/interference fromthe remained noise and the interference signal. A null RE may correspondto an empty RE (i.e., transmission power is 0 (zero)) in which no signalis transmitted by a base station. The null RE makes other base stationsexcept the corresponding base station easily measure a signal. In orderto measure an amount of noise/interference, it may use all of a CRS RE,a CSI-RS RE and a null RE. Or, a base station may designate REs to beused for measuring noise/interference for a UE. This is because it isnecessary to properly designate an RE to be used for measuringnoise/interference measured by the UE according to whether a signal of aneighbor cell transmitted to the RE corresponds to a data signal or acontrol signal. Since the signal of the neighbor cell transmitted to theRE varies according to whether or not synchronization between cells ismatched, a CRS configuration, a CSI-RS configuration and the like, thebase station identifies the signal of the neighbor cell and may be ableto designate an RE in which measurement is to be performed for the UE.In particular, the base station can designate the UE to measurenoise/interference using all or a part of the CRS RE, the CSI-RS RE andthe null RE.

For example, the base station may use a plurality of CSI-RSconfigurations and may be able to inform the UE of a CSI-RSconfiguration to be used for CQI feedback and a null RE position whileinforming the UE of one or more CSI-RS configurations. In order todistinguish the CSI-RS configuration to be used for CQI feedback by theUE from a null RE transmitted by zero transmission power, the CSI-RSconfiguration to be used for CQI feedback by the UE may correspond to aCSI-RS configuration transmitted by non-zero transmission power. Forexample, if the base station informs the UE of a CSI-RS configuration inwhich the UE performs channel measurement, the UE can assume that aCSI-RS is transmitted by non-zero transmission power in the CSI-RSconfiguration. In addition, if the base station informs the UE of aCSI-RS configuration transmitted by zero transmission power (i.e., nullRE position), the UE can assume that an RE position of the CSI-RSconfiguration corresponds to zero transmission power. In other word,when the base station informs the UE of a CSI-RS configuration ofnon-zero transmission power, if there exists a CSI-RS configuration ofzero transmission power, the base station can inform the UE of acorresponding null RE position.

As a modified example of the method of indicating a CSI-RSconfiguration, the base station informs the UE of a plurality of CSI-RSconfigurations and may be able to inform the UE of all or a part ofCSI-RS configurations to be used for CQI feedback among a plurality ofthe CSI-RS configurations. Hence, having received a request for CQIfeedback on a plurality of the CSI-RS configurations, the UE measures aCQI using a CSI-RS corresponding to each CSI-RS configuration and may bethen able to transmit a plurality of CQI information to the basestation.

Or, in order to make the UE transmit a CQI for each of a plurality ofthe CSI-RS configurations, the base station can designate an uplinkresource, which is necessary for the UE to transmit the CQI, in advanceaccording to each CSI-RS configuration. Information on the uplinkresource designation can be provided to the UE in advance via RRCsignaling.

Or, the base station can dynamically trigger the UE to transmit a CQIfor each of a plurality of CSI-RS configurations to the base station.Dynamic triggering of CQI transmission can be performed via PDCCH. Itmay inform the UE of a CSI-RS configuration for which a CQI is to bemeasured via PDCCH. Having received the PDCCH, the UE can feedback a CQImeasurement result measured for the CSI-RS configuration designated bythe PDCCH to the base station.

A transmission timing of a CSI-RS corresponding to each of a pluralityof the CSI-RS configurations can be designated to be transmitted in adifferent subframe or an identical subframe. If CSI-RSs according toCSI-RS configurations different from each other are designated to betransmitted in an identical subframe, it may be necessary to distinguishthe CSI-RSs from each other. In order to distinguish the CSI-RSsaccording to the CSI-RS configurations different from each other, it maybe able to differently apply at least one selected from the groupconsisting of a time resource, a frequency resource and a code resourceof CSI-RS transmission. For example, an RE position in which a CSI-RS istransmitted can be differently designated in a subframe according to aCSI-RS configuration (e.g., a CSI-RS according to one CSI-RSconfiguration is designated to be transmitted in an RE position shown inFIG. 7(a) and a CSI-RS according to another CSI-RS configuration isdesignated to be transmitted in an RE position shown in FIG. 7(b))(distinction using a time and frequency resource). Or, if CSI-RSsaccording to CSI-RS configurations different from each other aretransmitted in an identical RE position, the CSI-RSs can bedistinguished from each other by differently using a CSI-RS scramblingcode in the CSI-RS configurations different from each other (distinctionusing a code resource).

FIG. 11 illustrates an active antenna system (AAS).

In a wireless communication system after LTE Rel-12, the introduction ofthe antenna system utilizing the AAS has been discussed. Since eachantenna of the AAS corresponds to an active antenna including an activecircuit, an antenna pattern can be changed to adapt to a wirelesscommunication environment. That is, the AAS is considered as atechnology capable of reducing interference or performing efficientbeamforming.

Moreover, if the AAS is established in two dimensions (i.e., 2D-AAS), itis possible to adjust a beam direction at a main lobe of each antennanot only in the horizontal direction as in the related art but also inthe vertical direction in terms of the antenna pattern. Thus, the beamadaptation can be performed more efficiently in three dimensions.Further, a transmitted beam can be changed actively depending on alocation of a UE based on the beam adaptation. The 2D-AAS i.e., theantenna system having multiple antennas can be implemented by installingantennas in the vertical and horizontal directions.

When the above-described 2D-ASS is introduced, a large number ofantennas may be installed in a vertical antenna domain and thus thenumber of antennas is remarkably increased. To efficiently manage such alarge number of the antennas, reference signal (RS) design for measuringa channel at each antenna and feedback design for a UE to providefeedback of channel information between each antenna and the UE becomesvery important. The reason for this is that as the number of antennasincreases, RS overhead and feedback overhead generally increases eitherlinearly or exponentially.

In the current LTE system, REs (resource elements) amounting to thenumber of antenna ports are allocated for a CSI-RS in each PRB (physicalresource block) pair. If 64 antennas are used as shown in FIG. 11 and areference signal is designed to be similar to that of the current LTEsystem, 64 resource elements need to be allocated for the CSI-RS in eachPRB pair. In addition, in the case of the normal CP (cyclic prefix),considering that 168 resource elements are present in the PRB pair, toomany resource elements are currently used for the CSI-RS. Moreover,considering control channels and other reference signals, resourceelements that can be used for transmitting actual data are significantlyinsufficient.

In addition, a receiving end can measure channel information between atransmitting end and the receiving end using, for example, theabove-described reference signal and feed back channel state information(CSI) calculated based on the measured channel to the transmitting end.However, in the communication system to which the above-described 2D-AASis applied, since a large number of antennas are used, the number ofantennas required by the receiving end for the CSI feedback increases.For example, 4-bit precoding matrix indicator (PMI) information may berequired for 4 Tx antennas. Moreover, in the case of the 2D-AAS using 64antennas shown in FIG. 11, it is expected that, for example, 64-bit PMIinformation will be required. However, in this case, communicationefficiency may be degraded due to the large amount of channelinformation. Furthermore, the calculation amount of the PMI, CQI, and RIfor CSI may also be significantly increased, and it may become difficultfor the receiving end to generate the CSI within a limited time due tothe increased calculation amount. Further, the increased calculationamount may increase overall complexity of the receiving end.

To overcome the problems that occur when the 2D-AAS is applied, a methodfor using a precoded CSI-RS has been proposed. For example, precodingmay be applied to some or all of antenna elements of a BS to createmultiple beams. When the precoded CSI-RS is used, a UE may measure achannel using reference signals of the created multiple beams. Forexample, when N antenna elements are used, M beams are created usingprecoding, and N is greater than M, M resource elements may be used totransmit a reference signal in each PRB pair of a subframe reserved forreference signal transmission. Since in this case, the M REs are usedfor the RS transmission rather than N REs, the RS overhead may bereduced. In addition, since the amount of feedback related to the RS mayalso be reduced, overall signal overhead can be improved.

As an example of measuring a channel using the precoded CSI-RS, the BSmay create a plurality of elevation (vertical) beams, and the UE mayselect one among the plurality of elevation beams. For example, the BSwhere the 2D-AAS is applied may have antennas shown in FIG. 11.Specifically, as shown in FIG. 11, antennas in the 2D-AAS consists ofvertical antennas and horizontal antennas. For example, various types ofprecoding may be applied to antennas in column A of FIG. 11 to createvarious elevation beams, and reference signals may be transmitted to UEsthrough the created beams. The UE may calculate reference signalreceived power (RSRP) for each beam using a reference signal. Forexample, the UE may feed back information on upper N beams (where N is anatural number) with high RSRP to the BS. Thereafter, the BS may selectan elevation beam suitable for the UE based on the information receivedfrom the UE. In addition, the BS may transmit a reference signal forhorizontal antenna selection to the UE by applying precodingcorresponding to the selected elevation beam to each of the wholeantenna columns. The UE may measure a channel using the reference signalfor the horizontal antenna port where precoding of the selectedelevation beam is applied and then feedback CSI (e.g., RI, PMI, and CQI)for the measured channel to the BS.

During this process, only a vertical antenna domain is used in selectingan elevation beam. After the elevation beam is selected, only ahorizontal antenna domain is used to select a horizontal antenna.Accordingly, the RS overhead may be reduced compared to a method oftransmitting reference signals for all antenna elements. In addition,since CSI for only the horizontal antenna port is fed back to the BSafter the elevation beam selection, signaling overhead for feeding backthe CSI may also be reduced.

FIG. 12 illustrates an example of measurement using a precoded CSI-RS.

In FIG. 12, a serving cell 1210 of a UE 1230 and a neighboring cell 1220are illustrated. Referring to FIG. 12, the serving cell 1210 createselevation beams A and B, and the neighboring cell 1220 creates elevationbeams C and D. In FIG. 12, considering a distance and a beam direction,the beam B may be an elevation beam suitable for the UE 1230. Forexample, the beam B may be selected based on RSRP for the beams A and B.Meanwhile, the UE 1230 may experience interference caused by theneighboring cell 1220. For example, compared to the beam D, the beam Cmay cause higher interference to the UE 1230. Thus, if a BS of theneighboring cell 1220 uses the beam D instead of the beam C, theinterference to the UE 1230 may be reduced.

In the following embodiments, operations performed by a UE to reportreference signal received power (RSRP) or reference signal receivedquality (RSRQ) to reduce interference caused by a neighboring cell asdescribed above with reference to FIG. 12 will be described.

In the current LTE or LTE-A (advanced) communication system, a UE isconfigured to measure RSRP based on a DRS-CSI-RS (discovery referencesignal) using a common reference signal (CRS) or a DRS and transmit themeasured RSRP to a BS. In the following embodiments, it is assumed thata UE can measure RSRP without limitation on types of a BS and areference signal. For example, a UE may measure RSRP based on a CRS orDRS-CSI-RS. In addition, a UE may measure RSRP using a RRM-RS (radioresource management), which is a separate reference signal for RRMmeasurement.

In addition, in the LTE or LTE-A communication system, a UE isconfigured to feed back RSRP measured in each cell based on eventtriggering. More particularly, the UE may report an identifier (ID) of acell with high RSRP and the RSRP to discover an optimal cell based onevent triggering. For example, the UE may measure RSRP using aDRS-CSI-RS and then report the measured RSRP. In this case, a BS mayconfigure a DRS measurement timing configuration (DMTC) and inform theUE of information on resources for discovering a transmission point(TP). Moreover, for example, the BS may configure the DRS-CSI-RS andinform the UE of information on a reference signal for RSRP measurement.The UE retrieves a primary synchronization signal (PSS) in a resourceregion that is recognized through the DMTC to discover cells. The UE maymeasure the RSRP using DRS-CSI-RSs, which appear in the PSS after apredetermined offset, through DRS-CSI-RS configurations related to thesearch results. In this case, the UE may feed back, to the BS, IDs of upto three RSs among the DRS-CSI-RSs having RSRP values equal to orgreater than a prescribed threshold value and corresponding RSRP values.At this time, the UE may sequentially feed back information on thereference signals in descending order from the highest RSRP.

Hereinafter, embodiments related to RSRP measurement and/or reportingwill be described. In the following embodiments, the elevation beam maybe referred to as a vertical beam. The vertical beam means a beam ofwhich an incidence angle can be changed up and down by precoding in atwo-dimensional antenna plane, and the horizontal beam may mean a beamof which an incidence angle can be changed from side to side byprecoding in the two-dimensional antenna plane or antenna array.Moreover, 3D-MIMO may be referred to as a FD-MIMO (full dimensionalMIMO).

Embodiment 1

In embodiment 1, reference signals for RSRP measurement can be dividedinto two sets. For example, the reference signals can be divided into afirst set and a second set. In this case, a UE may respectively feedback RSRP for the first set and RSRP for the second set for an eventtriggering operation.

For example, the first set may include reference signals for a servingcell, and the second set may include reference signals for aninterference cell (e.g., neighboring cell). For example, in the case ofthe 3D-MIMO, reference signals for measuring RSRP for elevation beams ofthe serving cell may belong to the first set, and reference signals formeasuring RSRP for elevation beams of interference cells may belong tothe second set. Here, the interference cells may be a cell that does notuse the 2D-AAS. In this case, reference signals for measuring RSRP forhorizontal beams of the interference cells may also belong to the secondset. In addition, for example, the first and second sets may bedesignated by a BS in the same cell. The BS may transmit information ona reference signal to the UE. For example, the information on thereference signal may include a type of the reference signal, an ID ofthe reference signal, a reception interval of the reference signal,and/or information on designation of the reference signal sets.

For example, a DRS-CSI-RS may be used for the RSRP measurement. In thiscase, the UE may determine which one of the first and second sets eachDRS-CSI-RS belongs to on the basis of a physical cell ID of a DRS-CSI-RSconfiguration. In addition, the UE may feed back, to the BS, IDs ofupper N reference signals (where N is a natural number) among referencesignals that belong to the first set and have RSRP equal to or greaterthan a predetermined threshold and corresponding RSRP. Moreover, the UEmay feed back, to the BS, IDs of upper M reference signals (where M is anatural number) among reference signals that belong to the second setand have RSRP equal to or greater than a predetermined threshold andcorresponding RSRP. In this case, M and N may predefined so as to beequal to or different from each other. Alternatively, M and N may beconfigured either statically or semi-statically through higher layersignaling (e.g., RRC signaling).

In embodiment 1, the UE performs feedback for the first and second sets,respectively. As described above with reference to FIG. 12, the BS maydetermine an elevation beam of the serving cell suitable for the UEbased on the feedback for the first set. In addition, the BS may obtaininformation on a cell and an elevation beam which may cause seriousinterference to the UE based on the feedback for the second set.Moreover, the BS may perform scheduling coordination with a neighboringcell based on the obtained information.

Embodiment 2

In embodiment 2, an event triggering operation for RSRP feedback will bedescribed. According to the embodiment 2, the event triggering operationcan be defined as follows: “a UE feeds back IDs of at least N referencesignals (where N is a natural number) among reference signals havingRSRP equal to or smaller than a threshold and corresponding RSRP to aBS”. The event triggering operation of the embodiment 2 could beinterpreted to mean that information on lower N reference signals amongthe reference signals having the RSRP equal to or smaller than thethreshold is sequentially transmitted. The value of N may bepredetermined between transmitting and receiving ends or configuredeither statically or semi-statically through higher layer signaling(e.g., RRC signaling).

In the embodiment 2, the RSRP feedback method described above in theembodiment 1 can be applied. For example, a UE may divide referencesignals for RSRP measurement into two sets. For example, the first setmay include reference signals for a serving cell, and the second set mayinclude reference signals for an interference cell (e.g., neighboringcell). For example, in the case of the 3D-MIMO, reference signals formeasuring RSRP for elevation beams of the serving cell may belong to thefirst set, and reference signals for measuring RSRP for elevation beamsof interference cells may belong to the second set. Here, theinterference cells may be a cell that does not use the 2D-AAS. In thiscase, reference signals for measuring RSRP for horizontal beams of theinterference cells may also belong to the second set. In addition, forexample, the first and second sets may be designated by a BS in the samecell. The BS may transmit information on a reference signal to the UE.For example, the information on the reference signal may include a typeof the reference signal, an ID of the reference signal, a receptioninterval of the reference signal, and/or information on designation ofthe reference signal sets.

For example, the event triggering operation for the first set can bedefined as follows: “a UE feeds back IDs of at least N_(A) referencesignals among reference signals having RSRP equal to or greater than athreshold and corresponding RSRP to a BS”. In addition, for example, theevent triggering operation for the second set can be defined as follows:“a UE feeds back IDs of at least N_(B) reference signals among referencesignals having RSRP equal to or smaller than a threshold andcorresponding RSRP to a BS”. Here, N_(A) and N_(B) are natural numbers.In addition, the values of N_(A) and N_(B) may be predetermined betweentransmitting and receiving ends or configured either statically orsemi-statically through higher layer signaling (e.g., RRC signaling).

In the embodiment 2, the UE performs feedback for the first and secondsets, respectively. As described above with reference to FIG. 12, the BSmay determine an elevation beam of the serving cell suitable for the UEbased on the feedback for the first set. In addition, the BS may obtaininformation on an interference cell and an elevation beam which maycause less interference to the UE based on the feedback for the secondset. Moreover, the BS may perform scheduling coordination with aneighboring cell based on the obtained information.

Embodiment 2-1

Regarding the event triggering operation of the embodiment 2 (i.e., “aUE feeds back IDs of at least N reference signals among referencesignals having RSRP equal to or smaller than a threshold andcorresponding RSRP to a BS”), if a value of the measured RSRP is toosmall, the UE may not recognize the RSRP value. That is, the UE may notcalculate the RSRP for the corresponding reference signals. In addition,the UE may not also decode the IDs of the corresponding referencesignals.

Thus, RSRP equal to or smaller than a predetermined value X may bedefined as ‘a value that cannot be recognized’. In addition, the UE mayfeed back IDs of all the reference signals having the RSRP correspondingto ‘the value that cannot be recognized’ to the BS. For example, theDRS-CSI-RS may be used as a reference signal for the RSRP measurement.In this case, the UE may recognize IDs of configured DRS-CSI-RSs throughconfiguration information. In addition, the UE may feedback IDs of allthe reference signals having the RSRP defined as ‘the value that cannotbe recognized’ to the BS. For example, the UE may feedback IDs of Mreference signals having, the RSRP defined as ‘the value that cannot berecognized’ to the BS. If the number M of the reference signals havingthe RSRP defined as ‘the value that cannot be recognized’ is smallerthan the number N of reference signals of which RSRP and IDs need to bereported according to the event triggering operation, the UE may feedback IDs of (N-M) reference signals and corresponding RSRP to the BS.The predetermined value X may be determined between the BS and UE inadvance. Alternatively, it may be set to a different value per UEaccording to each UE's ability. Further, the predetermined value X maybe statically/semi-statically configured through higher layer signalingsuch as RRC signaling.

Embodiment 2-2

In the above-described embodiments, if the two event triggeringoperations, i.e., “a UE feeds back IDs of at least N reference signalsamong reference signals having RSRP equal to or greater than a thresholdand corresponding RSRP to a BS” and “a UE feeds back IDs of at least Nreference signals among reference signals having RSRP equal to orsmaller than a threshold and corresponding RSRP to a BS” aresimultaneously performed, it may be inefficient. For example, the BS mayestimate interference levels of a reference signal causing lessinterference and/or a reference signal causing strong interference andthen adjust scheduling based on the estimation. Thus, in theabove-described two event triggering operations, instead of feeding backIDs of reference signals that satisfy the conditions and correspondingRSRP to the BS, the UE may feed back only the IDs of the referencesignals that satisfy the conditions. The UE may be configured to alwaysperform such the event triggering operation (i.e., feedback of the IDsof the reference signals that satisfy the conditions). However, theabove-mentioned event triggering operation may be configured eitherstatically or semi-statically through higher layer signaling (e.g., RRCsignaling). In other words, feedback of the RSRP of the referencesignals that satisfy the predetermined conditions may also be configuredeither statically or semi-statically.

Embodiment 3

In embodiment 3, the event triggering operation for the RSRP feedbackwill be described. According to the embodiment 3, the event triggeringoperation can be defined as follows: “a UE feeds back IDs of at leastN_(S) reference signals (where N_(S) is a natural number) amongreference signals having RSRP equal to or greater than a first thresholdand corresponding RSRP to a BS and also feeds back IDs of at least N_(I)reference signals (where N_(I) is a natural number) among referencesignals having RSRP equal to or smaller than a second threshold andcorresponding RSRP to the BS”. The values of N_(S) and N_(I) may bepredetermined between transmitting and receiving ends or configuredeither statically or semi-statically through higher layer signaling(e.g., RRC signaling). Here, the first and second thresholds may bedifferent from each other.

In the embodiment 3, the RSRP feedback method described above in theembodiment 1 can be applied. For example, a UE may divide referencesignals for RSRP measurement into two sets. The first set may includereference signals for a serving cell, and the second set may includereference signals for an interference cell (e.g., neighboring cell). Forexample, in the case of the 3D-MIMO, reference signals for measuringRSRP for elevation beams of the serving cell may belong to the firstset, and reference signals for measuring RSRP for elevation beams ofinterference cells may belong to the second set. Here, the interferencecells may be a cell that does not use the 2D-AAS. In this case,reference signals for measuring RSRP for horizontal beams of theinterference cells may also belong to the second set. In addition, forexample, the first and second sets may be designated by a BS in the samecell. The BS may transmit information on a reference signal to the UE.For example, the information on the reference signal may include a typeof the reference signal, an ID of the reference signal, a receptioninterval of the reference signal, and/or information on designation ofthe reference signal sets.

For example, the event triggering operation for the first set can bedefined as follows: “a UE feeds back IDs of at least N_(S) referencesignals among reference signals having RSRP equal to or greater than athreshold and corresponding RSRP to a BS”. In addition, for example, theevent triggering operation for the second set can be defined as follows:“a UE feeds back IDs of at least N_(I) reference signals among referencesignals having RSRP equal to or smaller than a threshold andcorresponding RSRP to a BS”. Here, N_(S) and N_(I) are natural numbers.In addition, the values of N_(S) and N_(I) may be predetermined betweentransmitting and receiving ends or configured either statically orsemi-statically through higher layer signaling (e.g., RRC signaling).

In addition, as a single operation for the first and second sets, theevent triggering operation can be defined as follows: “a UE feeds backIDs of at least N_(S) reference signals among reference signals havingRSRP equal to or greater than a first threshold in a first set andcorresponding RSRP to a BS and also feeds back IDs of at least N_(I)reference signals among reference signals having RSRP equal to orsmaller than a second threshold in a second set and corresponding RSRPto the BS”.

Embodiment 3-1

Regarding one of the operations of the embodiment 3, i.e., “a UE feedsback IDs of at least N_(I) reference signals (where N_(I) is a naturalnumber) among reference signals having RSRP equal to or smaller than asecond threshold and corresponding RSRP to a BS”, if a value of themeasured RSRP is too small, the UE may not recognize the RSRP value.That is, the UE may not calculate the RSRP for the correspondingreference signals. In addition, the UE may not also decode the IDs ofthe corresponding reference signals.

Thus, RSRP equal to or smaller than a predetermined value X may bedefined as ‘a value that cannot be recognized’. In addition, the UE mayfeed back IDs of all the reference signals having the RSRP correspondingto ‘the value that cannot be recognized’ to the BS. For example, theDRS-CSI-RS may be as a reference signal for the RSRP measurement. Inthis case, the UE may recognize IDs of configured DRS-CSI-RSs throughconfiguration information. In addition, the UE may feedback IDs of allthe reference signals having the RSRP defined as ‘the value that cannotbe recognized’ to the BS. For example, the UE may feedback IDs of Mreference signals having the RSRP defined as ‘the value that cannot berecognized’ to the BS. If the number M of the reference signals havingthe RSRP defined as ‘the value that cannot be recognized’ is smallerthan the number N_(I) of reference signals of which RSRP and IDs need tobe reported according to the event triggering operation, the UE may feedback IDs of (N_(I)-M) reference signals and corresponding RSRP to theBS. The predetermined value X may be determined between the BS and UE inadvance. Alternatively, it may be set to a different value per UEaccording to each UE's ability. Further, the predetermined value X maybe statically/semi-statically configured through higher layer signalsuch as RRC signaling.

Embodiment 3-2

Regarding the event triggering operation of the embodiment 3, i.e., “aUE feeds back IDs of at least N_(S) reference signals (where N_(S) is anatural number) among reference signals having RSRP equal to or greaterthan a first threshold and corresponding RSRP to a BS and also feedsback IDs of at least N_(I) reference signals (where N_(I) is a naturalnumber) among reference signals having RSRP equal to or smaller than asecond threshold and corresponding RSRP to the BS”, if all of the IDs ofthe reference signals that satisfy the conditions and the correspondingRSRP are transmitted, it may be inefficient. Thus, in theabove-described event triggering operation, instead of feeding back theIDs of the reference signals that satisfy the conditions and thecorresponding RSRP to the BS, the UE may feed back only the IDs of thereference signals that satisfy the conditions. The UE may be configuredto always perform such the event triggering operation (i.e., feedback ofthe IDs of the reference signals that satisfy the conditions). However,the above-mentioned event triggering operation may be configured eitherstatically or semi-statically through higher layer signaling (e.g., RRCsignaling). In other words, feedback of the RSRP of the referencesignals that satisfy the predetermined conditions may also be configuredeither statically or semi-statically.

Embodiment 4

According to embodiment 4, the method proposed in the embodiment 2-1and/or the embodiment 3-1 can be defined as an event triggeringoperation to search for a reference signal having less interference. Forexample, the event triggering operation of the embodiment 4 can bedefined as follows: “when a UE cannot recognize IDs due to low RSRP, theUE feeds back IDs of all reference signals”. For example, when theDRS-CSI-RS is used as a reference signal for the RSRP measurement, theUE may obtain information on IDs of configured DRS-CSI-RSs fromconfiguration information. In this case, the UE may feed back all IDsthat the UE cannot recognize among IDs that the UE currently knows to aBS. In addition, the UE may define RSRP equal to or smaller than apredetermined RSRP value as ‘a value that cannot be recognized’, andthen determine IDs of reference signals having the RSRP corresponding to‘the value that cannot be recognized’ based on configuration informationon reference signals. Moreover, the UE may feed back the IDs of all thereference signals having the RSRP corresponding to ‘the value thatcannot be recognized’. Further, the UE may feedback a predeterminednumber of IDs among the IDs of the reference signals having the RSRPcorresponding to ‘the value that cannot be recognized’.

Embodiment 5

In the above-described embodiments, if a UE calculates and feeds backall RSRP for multiple reference signals, it may cause significant loadsto the UE. In addition, it may also significantly increase UEcomplexity. Accordingly, a time required for measuring received power ofa reference signal having RSRP equal to or smaller than prescribed RSRPmay be set to be lower than that required for measuring normal RSRP. Forexample, a measurement time of N ms may be configured to measure thenormal RSRP. When the UE intends to measure RSRP to determine referencesignals having RSRP equal to or smaller than the prescribed RSRP, the UEmay be configured to perform the measurement during M ms. Here, M may beset to be smaller than N. That is, considering that when RSRP is small,it is difficult to measure and calculate the RSRP, the measurement time(i.e., M ms) for the small RSRP may be set to be smaller than themeasurement time (N ms) for the normal RSRP. By doing so, the loads ofthe UE can be reduced.

FIG. 13 is a flowchart for explaining a method for feeding back areference signal according to an embodiment of the present invention.

Referring to FIG. 13, a UE is configured to measure RSRP [S1301] andfeed back information on reference signals that satisfy a predeterminedcondition based on the measured RSRP [S1302]. As described above, the UEmay be configured to measure a reference signal in a communicationenvironment where the 2D-AAS is applied. In addition, the steps of FIG.13 may be performed by a BS to select elevation beams for a UE. Thus,the UE may be configured to measure a reference signal for eachelevation beam where precoding is applied. In this case, each elevationbeam in the same cell may be created by applying precoding to antennaelements in the same column. Moreover, each elevation beam in the samecell may be configured to have a different elevation through precoding.That is, each elevation beam in the same cell may be configured to havea different angle in terms of an antenna plane in the vertical directionfrom the 2D-AAS antenna plane.

When measuring the RSRP [S1301], the UE may perform reference signalmeasurement without limitation on types of a BS and a reference signal.For example, the UE may perform measurement for reference signals of itsserving cell and/or neighboring cell (or interference cell). Inaddition, as described above in the embodiment 1, the UE may perform thereference signal measurement for reference signals in two sets.Moreover, the UE may be configured to have the same measurement timewith respect to all reference signals. However, as described above inthe embodiment 5, a measurement time for measuring a reference signalhaving RSRP equal to or smaller than a predetermined value may be set tobe smaller than that for measuring other reference signals. Furthermore,the RSRP measurement [S1301] may include reference signal reception.Additionally, the UE may receive reference signal configurationinformation to receive the reference signal.

Thereafter, the UE may feed back the information on the referencesignals that satisfy the predetermined condition based on the measuredRSRP [S1302]. In this case, the reference signals that satisfying thepredetermined condition may include a predetermined number of referencesignals having RSRP equal to or greater than a first threshold and/or apredetermined number of reference signals having RSRP equal to orsmaller than a second threshold as described above in the embodiments 1to 4. In addition, as described in the embodiments 2-1, 3-1, and 4, thepredetermined condition may imply a reference signal having RSRP thatcannot be recognized by the UE. Moreover, the UE may feed back an ID ofthe reference signal and the measured RSRP as information on thereference signal, and in some cases, the UE may be configured to feedback only the ID of the reference signal.

Although not shown in FIG. 13, the BS may determine an elevation beamsuitable for the UE and/or an elevation beam that causes interference tothe UE based on the feedback received from the UE. The BS may transmit areference signal to the UE by applying precoding for the elevation beamsuitable for the UE to antenna elements in each row. In this case, theUE may perform CSI feedback for horizontal ports, and the BS may selectvertical and horizontal beams suitable for the UE based on the CSIfeedback. In addition, the BS may mitigate the interference to the UE bysharing information on the elevation beam that causes the interferenceto the UE with a neighboring BS.

The above-described embodiments may correspond to combinations ofelements and features of the present invention in prescribed forms. And,it may be able to consider that the respective elements or features maybe selective unless they are explicitly mentioned. Each of the elementsor features may be implemented in a form failing to be combined withother elements or features. Moreover, it may be able to implement anembodiment of the present invention by combining elements and/orfeatures together in part. A sequence of operations explained for eachembodiment of the present invention may be modified. Some configurationsor features of one embodiment may be included in another embodiment orcan be substituted for corresponding configurations or features ofanother embodiment. And, it is apparently understandable that a newembodiment may be configured by combining claims failing to haverelation of explicit citation in the appended claims together or may beincluded as new claims by amendment after filing an application.

In this disclosure, a specific operation explained as performed by aneNode B may be performed by an upper node of the eNode B in some cases.In particular, in a network constructed with a plurality of networknodes including an eNode B, it is apparent that various operationsperformed for communication with a user equipment can be performed by aneNode B or other network nodes except the eNode B. ‘Base station (BS)’may be substituted with such a terminology as a fixed station, a Node B,an eNode B (eNB), an access point (AP) and the like.

Embodiments of the present invention may be implemented using variousmeans. For instance, embodiments of the present invention may beimplemented using hardware, firmware, software and/or any combinationsthereof. In case of the implementation by hardware, one embodiment ofthe present invention may be implemented by at least one of ASICs(application specific integrated circuits), DSPs (digital signalprocessors), DSPDs (digital signal processing devices), PLDs(programmable logic devices), FPGAs (field programmable gate arrays),processor, controller, microcontroller, microprocessor and the like.

In case of the implementation by firmware or software, one embodiment ofthe present invention may be implemented by modules, procedures, and/orfunctions for performing the above-explained functions or operations.Software code may be stored in a memory unit and may be then drivable bya processor.

The memory unit is located at the interior or exterior of the processorand may transmit and receive data to and from the processor via variousknown means.

It will be apparent to those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit and essential characteristics of the invention. Thus, theabove embodiments are to be considered in all respects as illustrativeand not restrictive. The scope of the invention should be determined byreasonable interpretation of the appended claims and all change whichcomes within the equivalent scope of the invention are included in thescope of the invention.

INDUSTRIAL APPLICABILITY

The above-described embodiments of the present invention can be appliedto various mobile communication systems.

What is claimed is:
 1. A method for feeding back reference signal information by a user equipment (UE) in wireless communication using a two-dimensional active antenna system (2D-AAS) including multiple antennas, the method comprising: receiving, from a base station (BS), a reference signal configuration including identifiers (IDs) of a plurality of reference signals included in a first reference signal set and a second reference signal set; receiving, from the BS, the plurality of reference signals, where a plurality of precoding is applied respectively; determining a reference signal that is not successfully received based on the reference signal configuration; measuring reference signal received power (RSRP) for each of the plurality of reference signals; and transmitting, to the BS, information on at least part of the first reference signal set and information on at least part of the second reference signal set based on the measured RSRP, and an ID of the determined reference signal, which is not successfully received, wherein precoding for the UE is determined based on the information on the at least part of the first reference signal set, and wherein interference information on the UE is determined based on the information on the at least part of the second reference signal set.
 2. The method of claim 1, wherein the first reference signal set is composed of reference signals of the BS, and wherein the second reference signal set is composed of reference signals of a neighboring BS adjacent to the BS.
 3. The method of claim 2, wherein different precoding is applied to each of the reference signals in the first reference signal set such that each of the reference signals in the first reference signal set has a different incidence angle in a vertical direction.
 4. The method of claim 1, wherein the information on the at least part of the first reference signal set includes an ID(s) of at least one reference signal having RSRP equal to or greater than a first threshold value in the first reference signal set, and wherein the information on the at least part of the second reference signal set includes an ID(s) of at least one reference signal having RSRP equal to or greater than a second threshold value in the second reference signal set.
 5. The method of claim 4, wherein the information on the at least part of the first reference signal set further includes RSRP for the at least one reference signal having the RSRP equal to or greater than the first threshold value in the first reference signal set, and wherein the information on the at least part of the second reference signal set further includes RSRP for the at least one reference signal having the RSRP equal to or greater than the second threshold value in the second reference signal set.
 6. The method of claim 1, wherein the information on the at least part of the first reference signal set includes an ID(s) of at least one reference signal having RSRP equal to or greater than a first threshold value in the first reference signal set, and wherein the information on the at least part of the second reference signal set includes an ID(s) of at least one reference signal having RSRP equal to or greater than a second threshold value in the second reference signal set.
 7. The method of claim 6, wherein a time period for measuring RSRP for each of a plurality of reference signals in the first reference signal set is longer than that for measuring RSRP for each of a plurality of reference signals in the second reference signal set.
 8. A user equipment (UE) in wireless communication using a two-dimensional active antenna system (2D-AAS) including multiple antennas, the UE comprising: a transceiver configured to transmit and receive signals; and a processor configured to: control the transceiver to receive, from a base station (BS), a reference signal configuration including identifiers (IDs) of a plurality of reference signals included in a first reference signal set and a second reference signal set, control the transceiver to receive, from the BS, the plurality of reference signals where a plurality of precoding is applied respectively, determine a reference signal that is not successfully received based on the reference signals configuration, measure reference signal received power (RSRP) for each of the plurality of reference signals, and control the transceiver to transmit, to the BS, information on at least part of the first reference signal set and information on at least part of the second reference signal set based on the measured RSRP, and an ID of the determined reference signal, which is not successfully received, wherein precoding for the UE is determined based on the information on the at least part of the first reference signal set, and wherein interference information on the UE is determined based on the information on the at least part of the second reference signal set.
 9. The UE of claim 8, wherein the first reference signal set is composed of reference signals of the BS, and wherein the second reference signal set is composed of reference signals of a neighboring BS adjacent to the BS.
 10. The UE of claim 9, wherein different precoding is applied to each of the reference signals in the first reference signal set such that each of the reference signals in the first reference signal set has a different incidence angle in a vertical direction.
 11. The UE of claim 8, wherein the information on the at least part of the first reference signal set includes an ID(s) of at least one reference signal having RSRP equal to or greater than a first threshold value in the first reference signal set, and wherein the information on the at least part of the second reference signal set includes an ID(s) of at least one reference signal having RSRP equal to or greater than a second threshold value in the second reference signal set. 